There are many thousands of ocean research buoys that presently bob up and down in the world's oceans, performing scientific research on temperature, salinity, global warming, ocean currents, and pollution. A graph of recent locations of these battery-powered buoys is shown in FIG. 1. At present, each buoy, which costs about $25K each, can travel between sea-level and about 1000 to 2000-m depths, and they are considered disposable after the batteries wear out, typically after some months of use.
There are also many gliding submersibles for the Navy and for research purposes. Each of these gliding vehicles cost about $100K, and the batteries last a period of months before these vessels are also disposed after the batteries run out. A typical gliding path is shown in FIG. 2 and may cover depths from sea level to about 700-m.
For most places in the world's temperate and tropical oceans, the temperature of the ocean at sea level (typically 15° C. to 30° C.) is significantly warmer than the ocean below about 500-m depth (typically 4° C. to 7° C.). There is a need to have a floater or glider that can use the oceans' temperature differences to generate electricity, thus allowing these submersibles to last a period of many years, instead of just months.
U.S. Pat. No. 5,291,847 discloses a submersible system that uses ocean temperature differences to supply buoyancy changes. This system uses a phase change material (PCM) that melts above 10° C. and freezes at temperatures below 10° C. The expansion upon melting causes a liquid to be compressed and stored, so that it can be released to an external bladder to create positive buoyancy when desired. Conversely, when the PCM freezes, it contracts and can accept fluid from the external bladder to cause the submersible to sink.
FIG. 3 (Jack A. Jones and Yi Chao, Novel Thermal Powered Technology for UUV Persistant Surveillance, Presentation to the ONR Joint Review of Unmanned Systems Technology Development, Panama City, Fla., Feb. 10, 2006, incorporated herein by reference in its entirety) shows an ocean thermal energy conversion (OTEC) system that uses hydraulic fluid from a phase change material (PCM) device, so that when the PCM compresses the liquid, it is stored in a high pressure bellows inside a pressurized nitrogen chamber. When electricity is desired, a valve is opened and the fluid pushes a piston that turns a generator to produce electricity.
In particular, PCM (10) expands when melted above 10° C., thus forcing out a hydraulic fluid (20) past a check valve (30). The hydraulic fluid (20) is then stored in a bellows (40) that is pressurized (e.g., a 200 Bar pressure) by gas in a chamber (50) external to the bellows (40).
When valve (60) is open, the hydraulic fluid (20) passes through and presses against a piston (70) that pushes geared rack and pinion teeth (80) against another set of gears (90) which will spin fast due to the gear ratio. A generator (100) is turned by the last gear in the gear train to produce power that charges a battery (110). Later, the hydraulic fluid (20) passes through valve (120) into a fixed volume chamber (130), thus allowing the piston (70) to return to its original position. When the PCM is cooled to below 10° C., it freezes and contracts, thus allowing the hydraulic, fluid to pass through check valve (140) into the volume (150) surrounding the PCM tubes (10).
The device of FIG. 3 proved to be too heavy, inefficient, complicated and expensive to fabricate. Commercially available, high-force, rack and pinion gears, as well as high-pressure pistons, as represented by (70), (80), (90), and (20) have a very large mass typically of over 15 kg. Furthermore, the required high ratio gears shown as (80), (90), and (100) typically have a very low efficiency of 0.5 or lower. Because the system has so many custom pieces that must be precisely aligned, it is both complicated and expensive to fabricate.